Amazon often discounts new books, both dead-tree and electronic. They slashed my book's price by a buck when it came out; the one-year anniversary is coming up soon; I don't know if it will return to the original $4.99 then. But that may explain the price increase that you mention. Also, if a book goes from indie to big-house published, the new publisher may increase the price.
Libby Cone
Author, War on the Margins: A Novel
http://tinyurl.com/6y973d